Field of the Invention
The present invention relates to an image forming apparatus.
Description of the Related Art
Cleaner-less systems (toner recycling systems) have previously been proposed for use in image forming apparatuses such as electrophotographic apparatuses or electrostatic recording apparatuses from the viewpoint of simplifying apparatus configuration and eliminating waste. In image forming apparatuses using a cleaner-less system, a drum cleaner that cleans the surface of the photosensitive drum following the transfer step is not provided, and untransferred toner on the photosensitive drum following the transfer step can be collected and reused as a result of carrying out so-called “simultaneous development and cleaning”. Simultaneous development and cleaning is carried out by collecting untransferred toner on the photosensitive drum following the transfer step with a fog removal bias (fog removal potential difference Vback, which is potential difference between direct current voltage applied to the developing apparatus and surface potential of the photosensitive drum) during development in the next and subsequent steps. According to this method, since untransferred toner is returned to the developing apparatus and reused in the next and subsequent steps, waste toner can be eliminated and the bother of performing maintenance can be reduced. In addition, as a result of being cleaner-less, there are also considerable advantages in terms of space, enabling the size of image forming apparatuses to be greatly reduced.
A contact DC charging system, in which the surface of the photosensitive drum is uniformly charged to inhibit the generation of ozone by allowing an electrically conductive charging apparatus to make direct contact instead of a corona charging device, has been proposed for use as a charging apparatus used in image forming apparatuses using a cleaner-less system (Japanese Patent Application Publication No. H10-213945). In this system, a DC bias is applied to a charging roller serving as a charging apparatus, the charging roller uniformly discharges while rotating and making contact with the surface of the photosensitive drum, and this causes the surface of the photosensitive drum to become uniformly charged. At the same time, residue toner remaining on the surface of the photosensitive drum after the transfer step when the residue toner has entered a charging zone, and the residue toner can be effectively collected in the developing apparatus due to the fog removal bias generated during development. In a contact DC charging system, there is the risk of residue toner unable to have been completely charged in the charging zone ending up adhering to the charging roller thereby resulting in defective charging. Therefore, as a result of driving and rotating the charging roller and photosensitive drum by providing a prescribed difference in circumferential velocity between the respective surfaces thereof, toner can be made to have negative polarity due to rubbing between the surface of the charging roller and the surface of the photosensitive drum. As a result, adhesion of toner to the charging roller can be inhibited.
In addition, a one-component contact development system has been proposed for use as the developing apparatus in image forming apparatuses using a cleaner-less system (Japanese Patent No. 4510493). In this system, development is carried out by retaining developer on the developing roller which contacts with the surface of the photosensitive drum. The developing roller is uniformly coated in a thin layer with toner supplied from the developing chamber. A regulating blade arranged in contact with the developing roller imparts a charge to the toner due to triboelectric charging between the regulating blade and the developing roller. As a result of DC bias being applied to the developing roller and the potential difference between the developing roller and the photosensitive drum, an electrostatic latent image of the surface of the photosensitive drum is developed. In addition, as described in Japanese Patent No. 2880356, the circumferential velocity of a developing roller is typically set higher than the circumferential velocity of a photosensitive drum to facilitate migration of residue toner charged by the charging roller from the drum to a developing roller.